Cancer is often the result of a complex set of genetic alterations that give aberrant cells the ability to undergo deregulated growth. Several different genetic alterations in mice have been shown to increase the incidence of tumor formation, but even in these cases, only subsets of cells within the organism give rise to tumors indicating that genetic changes beyond the initial insult are required to progress to tumor formation. We hypothesize that genetic changes in both the epithelial and mesenchymal compartments of the breast contribute to the initiation, growth and/or metastasis of breast cancer. The Transgenic and Embryonic Stem Cell (TG/ES) Core seeks to integrate the efforts of several investigators working to explore epithelial and stromal interactions and contributions leading to mammary carcinogenesis. The TG/ES Core has several specific goals related to the mammary oncogenesis project. The first of these goals is to develop transgenic mouse strains expressing Cre recombinase targeted to the mammary stromal fibroblasts under the influence of tissue-specific and inducible promoters. The TG/ES Core will also produce, import, rederive, distribute and cryopreserve strains of mice necessary to carry out individual Projects in the Program Project Grant. These include mice expressing Cre recombinase in the mammary epithelium and various mice carrying conditional (LoxP flanked) mutations detailed in individual Projects within the Program Project Grant.